# Special Relativity

## Experimental Verification

Like any scientific theory, the theory of relativity must be confirmed by experiment. So far, relativity has passed all its experimental tests. The special theory predicts unusual behavior for objects traveling near the speed of light. So far no human has traveled near the speed of light. Physicists do, however, regularly accelerate **subatomic particles** with large particle **accelerators** like the recently canceled Superconducting Super Collider (SSC). Physicists also observe cosmic rays which are particles traveling near the speed of light coming from space. When these physicists try to predict the behavior of rapidly moving particles using classical Newtonian physics, the predictions are wrong. When they use the corrections for Lorentz contraction, time dilation, and mass increase required by special relativity, it works. For example, muons are very short lived subatomic particles with an average lifetime of about two millionths of a second. However when they are traveling near the speed of light physicists observe much longer apparent lifetimes for muons. Time dilation is occurring for the muons. As seen by the observer in the lab time moves more slowly for the muons traveling near the speed of light.

Time dilation and other relativistic effects are normally too small to measure at ordinary velocities. But what if we had sufficiently accurate clocks? In 1971 two physicists, J. C. Hafele and R. E. Keating used atomic clocks accurate to about one billionth of a second (one nanosecond) to measure the small time dilation that occurs while flying in a jet **plane**. They flew atomic clocks in a jet for 45 hours then compared the clock readings to a clock at rest in the laboratory. To within the accuracy of the clocks they used time dilation occurred for the clocks in the jet as predicted by relativity. Relativistic effects occur at ordinary velocities, but they are too small to measure without very precise instruments.

The formula E=mc^{2} predicts that matter can be converted directly to energy. Nuclear reactions that occur in the Sun, in nuclear reactors, and in nuclear weapons confirm this prediction experimentally.

Albert Einstein's special theory of relativity fundamentally changed the way scientists characterize time and space. So far it has passed all experimental tests. It does not however mean that Newton's law of physics is wrong. Newton's laws are an approximation of relativity. In the approximation of small velocities, special relativity reduces to Newton's laws.

See also Cosmology; Quantum mechanics.

## Resources

### Books

Cutnell, John D., and Kenneth W. Johnson. *Physics.* 3rd ed. New York: Wiley, 1995.

Einstein, Albert. *Relativity.* New York: Crown, 1961.

Mould, R.A. *Basic Relativity.* Springer Verlag, 2001.

Hawking, Stephen. *Black Holes and Baby Universes and Other* *Essays.* New York: Bantam, 1993.

Schrödinger, Edwin. *Space-Time Structure,* Reprint Edition. Cambridge University Press, 2002.

Paul A. Heckert

K. Lee Lerner

## Additional topics

Science EncyclopediaScience & Philosophy: *Reason* to *Retrovirus*Special Relativity - History, Special Relativity, Space-time, Unusual Effects Of Motion, Speed Of Light Limit